The present invention relates to a process and a device for the preparation of flowable reaction mixtures from at least two reaction components. The components are mixed under high pressure. When the components are not being mixed, they are recirculated. The reversal back and forth between mixing and recirculating is effected by at least one flow reversing element which can be operated hydraulically.
The use of flow reversing elements in mix heads is generally known. Typical are the mixing devices or mix heads in which flow can be reversed by means of a flow reversing element, for example, those with a circulation or a so-called "straight" mix head. Depending on the construction, the flow reversing element can have various forms in conventional mixing devices or mix heads. In control piston mix heads (U.S. Pat. No. 3,706,515), the flow of the reaction components can be reversed by means of the control piston. If the reversing is effected via injection nozzles, the injection nozzles themselves represent the flow reversing element (U.S. Pat. No. 4,399,104). Finally, use of shut-off valves in the return (or recirculation) lines is also known. Such valves are typically located in the feed lines upstream of the mix head (U.S. Pat. No. 4,141,470).
These processes and devices are used in the polyurethane industry, and more particularly for the production of moldings in closed molds. During preparation of the reaction mixture (called the "shot" in the art), mixing disturbances occur at the start and end of the mixing operation because the injected jets of components initially stabilize at the start of the shot and then collapse at the end of the mixing operation. Even with high throughputs, small portions of unmixed or incompletely mixed components cause defects in the end product. If these defects appear on visible surfaces, the molding cannot be used or must at least be touched up. The shorter the flow reversing phases, the less such mixing disturbances occur.
One object of the present invention was to minimize the flow reversing phases mentioned using the simplest means possible.